Open the YouTube app
Tap the plus icon
Select Create a Short
Record or upload your Short
Tap Next
On the details screen, tap Edit thumbnail if available
Choose a frame from the video or upload a custom thumbnail if the option appears
Adjust the thumbnail frame as needed
Tap Done or Save
Add a title and other details
Tap Upload or Post
If Edit thumbnail is not available, set the thumbnail from YouTube Studio on desktop or use a frame from the Short itself